“The bureau’s spies first took an interest in the singer in 1943. Seeger, a 23-year-old army private at the time, had written denouncing a plan for mass deportation drafted by the California chapter of the American Legion, a veterans’ association. His angry letter prompted close scrutiny of his political views and associations by the bureau that ran on into the early 1970s. The suspicion was that Seeger, who died in early 2014, was a security risk with close connections to the Communist party. The FBI file on him has nearly 1,800 pages – 90 of them are still withheld for security reasons. An agent sent out to his Connecticut high school found evidence that he had worn ‘bohemian’ clothes.”
http://www.theguardian.com/music/2015/dec/20/fbi-spied-on-pete-seeger-20-years-communist-links